In her commentary on 1 Peter, Karen Jobes offered an original and astute defense of Petrine authorship against the common charge that its Greek is too refined for a Palestinian Jewish fisherman.
I recently ran across two examples that are analogous to her argument. There's a Chinese Christian blogger who's currently a student at an American seminary.
Normally he writes in a very educated English style. As a rule, his English style is more complex and erudite than many native English speakers.
I chalk that up to the fact that his English is heavily influenced by the kind of reading he does. Reading English-speaking theologians and Bible scholars. Academic prose.
Recently, however, I read two essays by him that contained a number of grammatical errors. That made me suspect that maybe his command of English is not as good as I supposed. Perhaps his usual routine is to write a draft, then have someone smooth out the grammatical infelicities. But on this occasion, for whatever reason, he didn't have that assistance.
In a related example, I was reading a Bayesian probability theorist (Tim Hendrix) commenting on atheist Richard Carrier. English is not Hendrix's native language. On the one hand, his comments contained some very technical vocabulary. On the other hand, they contained conspicuous grammatical errors (usually involving number agreement).
In both cases we have writers for whom English is a second language. Their vocabulary is more sophisticated than many native English speakers. Yet they commit syntactical blunders that a native English speaker would not.